Aggressive tumor cells share a number of characteristics with embryonic progenitors. During vertebrate development, multipotent precursor cells are gradually specified to particular fates through the autocrine or paracrine delivery of signaling molecules, and during cancer progression, malignant cells similarly release and receive cues that promote tumor growth and metastasis. Aggressive tumor cells, particularly melanoma cells, display stem cell-like plasticity as demonstrated by their molecular signature that signifies a dedifferentiated, multipotent plastic phenotype (i.e. one that is capable of responding to microenvironmental factors as well as influencing other cells via epigenetic mechanisms) (Bittner et al., 2000, Nature 406:536-540; Hendrix et al., 2003, Nat. Rev. Cancer 3:411-421). Furthermore, aggressive melanoma cells are capable of vasculogenic mimicry, i.e. they are able to form vasculogenic-like networks while simultaneously expressing genes associated with an endotheilial cell type. (Seftor et. al., 2002, Crit. Rev. Oncology Hematol. 44:17-27; Maniotis et. al., Am. J. Pathol. 155:739-752).
Previous studies capitalized on the similarities between cancer and stem cells by examining the ability of embryonic microenvironments to modulate tumor cell behavior (Pierce et al., 1982, Cancer Res. 42:1082-1087; Gerschenson et al., 1986, Proc. Natl. Acad. Sci. U.S. A 83:7307-7310; Lee et al., 2005, Dev. Dyn. 233:1560-1570; Mintz et al., 1975, Proc. Natl. Acad. Sci. U.S. A 72:3585-3589). For example, Pierce and colleagues reported that neural stage mouse embryos regulate neuroblastoma cells, and that embryonic skin inhibits melanoma growth ((Pierce et al., 1982, Cancer Res. 42:1082-1087; Gerschenson et al., 1986, Proc. Natl. Acad. Sci. U.S. A 83:7307-7310). Although studies have focused on the role of embryonic signals in the regulation of tumor cells, few have utilized embryonic models as a tool to discover molecular mechanisms by which cancer cells modulate their microenvironment and the resulting reciprocal interactions.
One of the major factors contributing to the plasticity of stem cells is Nodal. Nodal is a highly conserved morphogen belonging to the transforming growth factor beta (TGFβ) super family (Schier et al., 2003, Annu. Rev. Cell Dev. Biol. 19:589-621). By acting as an organizing signal before gastrulation, Nodal initiates embryonic axis formation, and previous studies demonstrated that the ectopic expression of Nodal induces mesendodermal fates in ectopic positions (Whitman, 2001, Dev. Cell 1:605-617; Schier, 2003, Annu. Rev. Cell Dev. Biol. 19:589-621; Iannaccone et al., 1992, Dev. Dyn. 194:198-208; Smith, 1995, Curr. Opin. Cell Biol. 7:856-861; Zhou et al., 1993, Nature 361:543-547; Rebagliati et al., 1998, Proc. Natl. Acad. Sci. U.S. A 95:9932-9937; Toyama et al., 1995, Development 121:383-391).
Activation of Nodal includes binding to the co-receptor Cripto and subsequent phosphorylation of the type I and type II activin-like kinase receptors (ALK). In turn, SMAD2 and SMAD3 are activated (Lee et. al., 2006, Nature Medicine 12:882-884). Furthermore, human embryonic stem cells express Nodal and secrete endogenous inhibitors of Nodal such as Le A/B (Besser, D., 2004, J. Biol. Chem. 279:45076-45084). Lefty A and Lefty B, human homologs to murine Lefty 2 and Lefty 1, respectively, are separated by approximately 50 kb on chromosome 1q42 and are 96% identical to each other (Kosaki et. al., 1999, Am. J. Hum. Genet. 64:712-21). Lefty A and Lefty B are members of the TGFβ superfamily, and are considered one of the powerful inhibitors of Nodal.